Carbonyl compounds alcohol oxidations, palladium acetate

Carbonyl Compounds by Oxidation of Alcohols and Aldehydes. Salts of palladium, in particular PdCl2 in the presence of a base, catalyze the CCI4 oxidation of alcohols to aldehydes and ketones. Allylic alcohols carrying a terminal double bond are transformed to 4,4,4-trichloro ketones at 110 °C, but yield halo-hydrins at 40 °C. These can be transformed to the corresponding trichloro ketones under catalysis of palladium acetate (eq 56). The latter transformation could be useful for the formation of ketones from internal alkenes provided the halohydrin formation is regioselective. 

Several examples have been reported of the use of palladium-mediated oxidation reactions of alcohols and alkyl halides. Palladium(II) acetate in the presence of iodobenzene converts primary and secondary alcohols into carbonyl compounds under solid-liquid two-phase conditions [20], However, other than there being no further oxidation to carboxylic acids, the procedure has little to commend it over other methods. It is relatively slow with reaction times in the order of 2 days needed to achieve yields of 55-100%. 

The mechanism of the Zn chloride-assisted, palladium-catalyzed reaction of allyl acetate (456) with carbonyl compounds (457) has been proposed [434]. The reaction involves electroreduction of a Pd(II) complex to a Pd(0) complex, oxidative addition of the allyl acetate to the Pd(0) complex, and Zn(II)/Pd(II) transmetallation leading to an allylzinc reagent, which would react with (457) to give homoallyl alcohols (458) and (459) (Scheme 157). Substituted -lactones are electrosynthesized by the Reformatsky reaction of ketones and ethyl a-bromobutyrate, using a sacrificial Zn anode in 35 92% yield [542]. The effect of cathode materials involving Zn, C, Pt, Ni, and so on, has been investigated for the electrochemical allylation of acetone [543]. 

The oxidation of a, -unsaturated carbonyl compounds under the usual conditions in DMF using PdCl2/CuCl/02 is very slow. However, regioselective oxidation of a, -unsaturated esters to 3-keto esters (equation IS), and a,3-unsaturated ketones to 1,3-diketones (equation 16) proceeds with NazPdCU in solvents such as S0% acetic acid, isc ropyl alcohol, and NMP. r-Butyl hydroperoxide and hydrogen peroxide are used as the reoxidants of the reduced palladium. The reaction proceeds slowly at room temperature but smoothly between SO and 80 C. Some typical examples of this process are shown in Table 1. 

Addition of tributylstannyl-lithium to crotonaldehyde and protection of the resulting alcohol with chloromethyl methyl ether gives the stannane (192), which reacts with both alkyl and aryl aldehydes RCHO to form specifically the t/rr o-hydroxy-enol ethers (193). These latter compounds have been used to prepare tra/i5-4,5-disubstituted butyrolactones by hydrolysis and subsequent oxidation. Palladium-catalysed carbonylation of RX in the presence of organotin species constitutes a useful synthesis of unsymmetrical ketones, and in the example reported this year RX is an arenediazonium salt. The reaction, which is basically an aromatic acylation, proceeds in good to excellent yield. Another Pd-catalysed reaction of aromatics, this time aryl bromides, is their reaction with acetonyltributyltin (194), prepared from methoxytributyltin and isopropenyl acetate, to give the arylacetones (195). ... 

Allylic Oxidation. The Wacker reaction and related palladium-catalyzed oxidations which proceed via nucleophilic attack on coordinated alkene have been widely practiced in industry to produce acetaldehyde, acetone, and vinyl acetate. An alternative pathway is available to alkenes in the coordination sphere of palladium(ll) complexes, which could lead to another important family of oxidation products. Insertion into the allylic C-H bond of 1-alkenes gives TT-allyl complexes which, on attack by external nucleophiles, would produce a family of allylic oxidation products including a,fi-unsaturated alcohols, carbonyl compounds, and carboxylic acids. Electron- withdrawing anionic ligands such as trifluoroacetate enhance the ability of the palladium center to insert into C-H bonds in this manner [26] (Fig. lA). Catalytic conversion of propylene to allyl acetate has been achieved in high selectivity in the presence of catalytic quantities of palladium(ll) trifluoroacetate [27]. 

The reaction of alcohols with CO was catalyzed by Pd compounds, iodides and/or bromides, and amides (or thioamides). Thus, MeOH was carbonylated in the presence of Pd acetate, NiCl2, tV-methylpyrrolidone, Mel, and Lil to give HOAc. AcOH is prepared by the reaction of MeOH with CO in the presence of a catalyst system comprising a Pd compound, an ionic Br or I compound other than HBr or HI, a sulfone or sulfoxide, and, in some cases, a Ni compound and a phosphine oxide or a phosphinic acid.60 Palladium(II) salts catalyze the carbonylation of methyl iodide in methanol to methyl acetate in the presence of an excess of iodide, even without amine or phosphine co-ligands platinum(II) salts are less effective.61 A novel Pd11 complex (13) is a highly efficient catalyst for the carbonylation of organic alcohols and alkenes to carboxylic acids/esters.62... 

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